It is a problem that has puzzled some of history's greatest minds, and even
occupied the not inconsiderable thoughts of Charles Darwin: do people blush
in the dark?

Blushing has been described as the most peculiar human expression, because so many questions exist as to how and why our faces suddenly flush with colour.

One age-old question has been whether we make the distinctive facial expression when alone, or in the dark, because there is no one around to notice it, but historically one fundamental issue made the problem difficult to solve.

Writing in the late 18th century the German scholar Georg Christoph Lichtenberg observed: "The question whether young women blush in the dark is a very difficult one; at least, one that cannot be settled by light."

Even Charles Darwin was unable to solve the mystery, although he did suggest an answer in The Expression of the Emotions in Man and Animals: "Several ladies, who are great blushers, are unanimous in regard to solitude; and some of them believe that they have blushed in the dark ... I have no doubt that this latter statement is correct."

Fortunately for modern scientists, the development of heat-sensitive cameras has opened up the possibility of solving the mystery once and for all.

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Researchers from the University of Erfurt, Germany, and the Dutch Academy of 'Pataphysics, Amsterdam, decided to find an answer, noting that although the technology is not new, "apparently and surprisingly the conclusive test has not been done".

Denise Wächter and Wolter Seuntjens used a Testo-875 thermogenic camera in a pitch dark room to film the face of a 31-year-old woman who was specially selected for her "talent to blush easily".

After allowing her to acclimatise to the temperature of the room, they made one measurement before asking her to describe an embarrassing story that had happened to her.

The women told researchers she felt as if she was blushing, and a second measurement revealed that the amount of heat in her face had increased significantly, showing that blood had rushed to her cheeks.

Reporting their findings in the Annals of Improbable Research, they wrote: "By showing that people can blush in the dark we have finally solved this age-old problem."

The findings could shed light on the purpose of blushing, which is most likely a social behaviour similar to laughing or yawning, they explained.

Further research would be needed into how often people blush alone, relative to how often they do so in a group, in order to determine how "social" a gesture it is, however.

"Perhaps the individual's propensity to blush when alone may prove to be a diagnostic and prognostic personality trait. In the case of blushing this trait might indeed be a measure for 'modesty'," they said.

The Annals of Improbable Research is a journal dedicated to quirky studies which "make people laugh, then think".

Its annual award ceremony, in which the authors of the most improbable research are awarded "Ig Nobel Prizes" will take place next week at Harvard University.

Last year's winning studies included an analysis of why ponytails curl at the end, an experiment demonstrating that buildings look smaller if you lean to the left, and an experiment which demonstrated that meaningful brain activity could be found in a dead salmon.